Nonvolatile memory cells that are electrically programmable and erasable can be realized as charge-trapping memory cells, which comprise a memory layer sequence of dielectric materials with a memory layer between confinement layers of dielectric material having a larger energy band gap than the memory layer. The memory layer sequence is arranged between a channel region within a semiconductor body and a gate electrode provided to control the channel by means of an applied electric voltage. Examples of charge-trapping memory cells are the SONOS memory cells, in which each confinement layer is an oxide and the memory layer is a nitride of the semiconductor material, usually silicon (U.S. Pat. No. 5,768,192 and U.S. Pat. No. 6,011,725, both of which are incorporated herein by reference).
Charge carriers are accelerated from source to drain through the channel region and gain enough energy to be able to penetrate the lower confinement layer and to be trapped within the memory layer. The trapped charge carriers change the threshold voltage of the cell transistor structure. Different programming states can be read by applying the appropriate reading voltages.
A publication by B. Eitan et al., “NROM: a Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell” in IEEE Electron Device Letters, volume 21, pages 543 to 545 (2000), incorporated herein by reference, describes a charge-trapping memory cell with a memory layer sequence of oxide, nitride and oxide which is especially adapted to be operated with a reading voltage that is reverse to the programming voltage (reverse read). The oxide-nitride-oxide layer sequence is especially designed to avoid the direct tunneling regime and to guarantee the vertical retention of the trapped charge carriers. The oxide layers are specified to have a thickness of more than 5 nm.
The memory layer can be substituted with another dielectric material, provided the energy band gap is smaller than the energy band gap of the confinement layers. The difference in the energy band gaps should be as great as possible to secure a good charge carrier confinement and thus a good data retention. When using silicon dioxide as confinement layers, the memory layer may be tantalum oxide, cadmium silicate, titanium oxide, zirconium oxide or aluminum oxide. Also intrinsically conducting (non-doped) silicon may be used as the material of the memory layer.
The memory layer sequence of a charge-trapping memory cell is composed of dielectric materials. Therefore, it is possible to subdivide the memory layer into separate regions located at the source/drain regions of the memory cell transistor (for example U.S. Pat. No. 6,335,554 B1, incorporated herein by reference). In this way, it is possible to store two bits of information by an application of programming currents in two opposite directions. The function of the source region and the drain region is interchanged so that channel hot electrons are alternatively injected into one of the two separate memory layer regions. The programmed status of the memory cell can be read for the two charge-trapping regions separately. Thus, two bits of information can be stored separately in each transistor memory cell. The stored information is maintained for more than ten years if the memory layer and confinement layers are produced accordingly.
The amount of charge carriers which are trapped in the course of the programming process is not strictly fixed. Therefore, the threshold voltage of the cell transistor may display certain variations so that the actually sensed value deviates from the average value. The distributions of the threshold voltages of the programmed “0” and the programmed “1” should be as narrow as possible and should especially not overlap too much. Ideally, a complete separation of the ranges of the values of the threshold voltages for the two different programmed states is realized so that they do not overlap at all. To obtain this, the relative position of the charge-trapping regions and the adjacent junction of the corresponding source/drain region is critical.